Method of electrolytic coloring of oxide layers on aluminum and aluminum base alloys

ABSTRACT

Method for electrolytic coloring of oxide layers produced by anodisation on aluminum and aluminum alloys, in at least two color sequences, consisting in using one coloring electrolyte which contains at least two metal salts of which each one renders different colors to the others, for example copper and tin salts, the different color sequences being obtained by adjusting the electrolysis AC-current voltage adequately. The counter electrode may be of the same metal that is dissolved in form of a salt in the electrolyte.

v it Eli,

inger et al.

[451 Jan. 2 1974 of Neuhausen; Fritz Sehneeberger, Schaffhausen, all of Switzerland [73] Assignee: Swiss Aluminium Ltd., Chippis,

Switzerland [22] Filed: Apr. 1, 197 1 [2 1 Appl. No.: 130,484

[30] Foreign Application Priority Data Apr. 2, 1970 Switzerland 4867/70 [52] US. Cl 204/35 N [51] int. Cl C23b 9/02 [58] Field of Search 204/58, 35 N [56] Reterences Cited UNITED STATES PATENTS 3,382,160 5/1968 Asada et al. 204/58 3,664,932 5/1972 Pattie 204/58 3,654,100 4/1972 Nagai et a] 204/58 FOREIGN PATENTS OR APPLICATIONS 4/1963 Canada 204/58 69,930 l/l946 Norway 204/58 120,248 9/1970 Norway 204/58 OTHER PUBLICATIONS Handbook of Chemistry and Physics, 32nd ed. 1950, p. 1521 Modern Electroplating by Lowenheim, 1963, p 468.

Primary ExaminerJohn H. Mack Assistant Examiner-R. L. Andrews [5 7] ABSTRACT Method for electrolytic coloring of oxide layers produced by anodisation on aluminum and aluminum alloys, in at least two color sequences, consisting in using one coloring electrolyte which contains at least two metal salts of which each one renders different colors to the others, for example copper and tin salts, the different color sequences being obtained by adjusting the electrolysis AC-current voltage adequately.

The counter electrode may be of the same metal that is dissolved in form of a salt in the electrolyte.

7 Claims, No Drawings 1 METHOD OF ELECTROLYTIC COLORING OF OXIDE LAYERS ON ALUMINUM AND ALUMINUM BASE ALLOYS Many processes are known by which oxide layers obtained by anodising aluminium are coloured by subsequent electrolytic treatment. The aluminium piece being connected as an electrode, the essentially colourless layers are thereby subjected to an alternating current treatment in acidiferous solutions containing one or more metal salts and any addition agents as may be required. As is known, those metals which in a compound form a colouring constituent of the colouring electrolyte may also be used as material for the counter electrodes.

For example, salts of the following elements: Fe, Co, Ni, Mn, Cr, Bi, As, Sb, Sn, Ag, Cu, Au, Cd, Mo, Ti, Ca, Mg, V, Pb and Zn were accordingly proposed as components of the colouring electrolytes, the various elements and groups of elements as, for example, 80,", N Cl, oxyacids such as from Te, B, Cr and P and organic acids and acid anions such as acetates, tartrates, oxalates and citrates being suggested as anionproducing components of such electroytes. NH amino and imino groups are also known as additional cationforming constituents of such electrolytes.

Metals, metal hydroxides, metal oxides and metal salts have previously been mentioned as effective colouring substances which penetrate the oxide layers under these conditions.

With all these known methods a typical colour is determined and produced by means of the bath composition and especially by choosing the specific colouring metal salt, the depth of colour being influenced by electrical conditions (voltage/current intensity) and/or the time of exposure to these electrical parameters of the piece to be coloured.

In this way, with a pre-det'ermined colour electrolyte, one can obtain only a limited variation of colours. The conventional colouring methods have therefore the disadvantage that for each choice of colour an independent colouring bath or a change in bath composition or an exchange of electrolytes is required. A further disadvantage is the insufficient dispersibility (or scattering) of many electrolytes which, without auxiliary means, results in irregular colours being obtained. This is particularly noticeable in everyday practice. An insufficient dispersibility (or scattering) of the electrolyte especially shows its effects in respect to objects to be coloured whereby indentations therein are coloured too light and the edges of large dimensioned sheets are too dark. To obviate this, usual countermeasures are applied such as covering means, auxiliary electrodes, suitable electrode arrangements, screens, made partly of metal or plastic and certain stepwise changes of the current conditions, as for example, current surges at the commencement of colouring. These means are well known in the galvanizing techniques.

Pursuent to research work with the numerous metal salts applicable for colouring purposes and from the known anions likely to be used for electrolytic colouring, it was surprisingly discovered in accordance with the present invention, that it is possible to determine and apply at least two kinds of metal ion-containing colour electrolysis compositions with which, and in a relatively wide range of applications, one is able to produce, in one and the same bath, at least two colour 2 scales in previously anodised layers on one and same alloy.

in the sense of the invention it is understood that two such colour scales are those which are not producable within the pre-anodised layers by applying only one of the plurality of colouring metal salts used according to the invention.

Consequently, the present invention concerns a process for the colouring of anodised, protective layers on aluminium and its alloys in a variety of colours by means of an electrolytic treatment of the protective layer with alternating current, using counter electrodes in acidiferous colouring metal salt electrolytes in which there is used a colouring electrolyte containing at least two colouring metal salts, each salt establishing its own variety of colours and that the said variety of colours are established during the said electrolytic treatment in one and the same colouring electrolyte by means of differing heights in electrical voltage.

By the method according to the invention the colour to be produced in the anodised layer is selectable from two colour scales obtainable in one and the same colour electrolyte containing at least two colouring metal salts by adjusting the electrical current/voltage conditions and duration of treatment, whereby changing these conditions leads to the production of the other colour scale. However, not every combination of two colouring metal salts in a colouring electrolyte has the property to produce two different colour scales in preanodised layers. For example, the conventional colouring electrolytes comprising Ni or C0 salts, or the Sn salts, can only render one colour sequence. As a rule however, in the process according to the invention, a successfully operating electrolyte comprising a combination of two colour metal salts is obtained if two colouring metal salts are selected whereby each renders its own individual colour and when those conventional parameters of anion kinds and pH values (easily discoverable by routine methods) are applied by which each of the two said metal salts of the colouring electrolyte can produce its own colouring effects in anodised layers. The latter condition is quite easily accomplished provided that each of the two metal salts is individually colour-electrolytic effective in equal or similar pH ranges.

In some cases however, such suitable parameters can also be determined and applied by selecting and adding suitable anion-producing means or coloured complex compound materials or other means including buffer substances which all said means result in a modification in the composition of the bath thereby making it suitable for the formation of two different coloured pigments.

During the tests which led to the present invention the following groups of metal salts were established and are arranged according to their various self colours:

the

Color Group: brownish reddish yellowish bluish Ni salt Cu salt Ag salt Sn salt Cd salt Mo salt Co salt Consequently a double salt metal electrolyte rendering two different colour sequences can be obtained by using for example Sn salt and Cu salt, or Ag salt and Cu salt, or Cd salt and Sn salt, as well as of the conventional selection and application of suitable anions or buffer substances relative to the pH value and/or suitable additions, as for example NH, salts of the prior art.

According to the invention the one kind of metal ion and also its degree of valency is essentially responsible for one colour scale, the other metal ion type for another, similarly obtainable, colour scale. Consequently concentrations of kinds of metallic ions have to be selected by experiment, according to the aim to be achieved, since any excessively low concentration or activity of one metallic ion kind makes the production of two different colour scales according to the invention by means of one and the same colour electrolyte impossible.

The time program for applying the selected voltage steps, the duration as such, and the final voltage value, all influence the colours of the two different colour scales produced according to the invention and can easily be determined for the present purposes by tests. The length of treatment time influences the depth of colour although it noticeably less influences the kind of obtainable colour scale, similarly to the known methods for procuring only one colour sequence.

Likewise, the concentration of further additions, as for example, acids, determines the appearance of both colour scales. According to the invention it is in any case essential that with one and the same electrolyte containing at least two different colouring metal salts at least two different continuous colour sequences can be obtained on one and the same alloy by the selection of suitable electrical conditions.

According to the foregoing, it is in many cases possible and practical to add further conventional addition agents to baths producing at least two colour sequences according to the invention, and to apply further means for example, for the stabilisation of the valency of metallic ions or the pH value of the electrolyte.

The expert may select, for the method according to the invention, the most practicable material for the counter electrode as for example graphite, aluminium or metal carbides or for example a counter electrode of a metal which represents, as a composition, part of the colouring electrolyte. The suitable adjustment of pl-l value and temperature of the colouring electrolyte will also be selected by means of tests or by theoretical calculations for each specific combination of metal salts which will render the desired two colour sequences.

Generally, anodised layers produced by any conventional methods may be coloured according to the invention process. Excellent results were found for example on anodised layers produced in baths which contain, as main component, acids of the sulfuric, chromic and sulfamic group.

A preferred embodiment of the method according to the invention rendering two characteristic colour scales was found when a colouring electrolyte containing at least two kinds of differently colouring ions of the metals in the Cu, Ag, Cd, Mn. Sn, Ni, Co and Mo group and anion-forming acids or acid radicals containing one acid or one salt of the citric, tartaric, maleic, succinic, fumaric, adipic, acetic, lactic, malonic, phthalic, sulfuric, selenic, phosphoric, fluoboric, formic, sulfamic, sulfosalicylic, sulfanilic and phenosulfonic acids group.

Example 1 Color Scale I Alternating Current Coloring time (minutes) Voltage Yellow light yellow 8 l medium yellow 8 5 dark yellow 8 10 Color Scale ll Brown light brown 14 l medium brown 14 2.5 dark brown l4 l0 In this Cu/Ag containing electrolyte the sulfuric acid may be totally or partially replaced by other acids, for example, by tartaric, oxalic, succinic, phthalic, sulfamic, sulfosalicylic, citric acids and otherst According to a further preferred embodiment of the invention, colouring electrolytes are used which, as anion-producing acids or acid radicals, contain at least two different acids or their salts from the group of acids specified in the previous paragraph. Another, especiallyadvantageous embodiment of the present invention consists of utilizing a colouring electrolyte containing at least copper and tin ions, and as anion-producing acids at least one acid or one salt of the citric, tartaric, maleic, succinic, fumaric, adipic, acetic, lactic, malonic, phthalic acids group (1), and at least one additional acid from the sulfuric, selenic, phosphoric, fluoboric, formic, sulfamic, sulfosalycilic, sulfanilic, phenolsulfonic acids group (2).

.Such a colouring electrolyte may contain for example:

0.5 up to 100 g/l tin-ll-sulfate 0.5 up to g/l CuSO 5 H O 0.5 up to g/l citric acid 0.5 up to 100 g/l sulfuric acid conc.

For some architectural purposes for example, it is very advantageous when the colouring electrolyte contains the following concentrations: Preferably:

5 up to 20 gll tin-ll-sulfate 13 up to 17 gll Sup to 10 gll CUSO4-5H2O Sup to 10 gl] 5 up to 20 g/l citric acid 8 up to 12 gll 5 up to 2 0 gll sulfuric acid cone. 8 up to 12 gll As regards the bath composition, it appeared that not every organic acid other than citric acid, or one of those organic acids mentioned in the acid group.( 1 maybe .used with success. :For instance, when maleic acid and formic acid were used instead of citric acid under otherwise same conditions, only one colour scale was obtained. Consequently, formic acid is not able to regularly assume itself as substitute of anacid of the above mentioned first acid group. Neither can the acids of acid group (2:), i.e. sulfuric, selenic, phosphoric acids etc., mentioned as component of an Sncontaining type of electrolyte, be replaced by any other organic acid. The following organic acids, under observance of all other necessary requirements, render no' colouring or only one colour sequence: hydrofluoric acid, nitric acid, chromic acid.

Example 2 Preanodization for the production of the layer to be coloured:

Aluminium sheet -300 X 600 X l mm of alloy AlM- g1.5 Sulfuric .acid l65 g/l; Temperature 18 i 1C Current density -1.5 Amp/dm Voltage: 16-18 V direct current Countereleetrode: lead. Rinse in water prior to colouring. r Colouring in two different sequences:

1. bronze, beige shades through blackish brown to black 2. reddish-beige shades through wine red to black whereby the reddish character becomes more conspicuous when viewed from a reflecting angle.

The colouring electrolyte contains:

g/l tin-lrl' sulfate 10 g/l citric acid 10 g/l sulfuric acid conc. The pH value amounts. to approximately 1.3 Bath temperature: 20C Counterelectrodes: graphite rods of 5 mm 4) As apparent from the chart, one obtains a reddish colour scale with thiscolouring-electrolyte at lower voltage and a beige colour scale with higher voltages.

This. example shows the excellent scattering power (penetration power) of the electrolytes used in the colouring process according to the invention:

A U-shaped section, made of an AlMgSi alloy having as inner dimensions mm deep and 20 mm wide as preanodised as specified in Example 2. Subsequently the piece was coloured in the electrolyte according to Example 2 by applying an AC current of 14 volt during 5 minutes. The work piece resulted in being uniformly coloured in dark brown, even in the deepest parts. This proves good penetration power of the electrolyte.

The fact and general teaching brought by the invention whereby according to which, it is possible to produce not only one but also at least two different colour sequences within preanodised layers by means of one colouring electrolyte containing at least two colouring metal salts, is of significant importance for industrial applications since, with the use of only one colouring installation at least two colour sequences can be produced which up to now required several installations with different colouring electrolytes or at least the exchange of the colouring electrolytes in the same installation. The method of colouring according to the invention has the advantage of using electrolytic baths with a good scattering power which means that the colours are uniform, well reproducable and independent of the configuration of the work pieces and of their arrangement relative to the counterelectrodes.

What we claim is:

1. Method for colouring previously anodised protective layers on aluminium and aluminium base alloys in various colour sequences by means of electrolytic treatment of the protective layers with alternating current under the application of counter electrodes in acidiferous electrolytes containing colouring-metal salts,

in which a single colouring electrolyte is used containing salts of at least two colouring metals of which each one renders a different colour from the other or others and wherein in this same electrolyte these different colours are obtained by varying the value of the electrical voltage for the electrolytic treatment.

2. Method according to claim 1, in which said colour- .ing electrolyte contains at least two kinds of differently colouring ions of the metals in the Cu, Ag, Cd, Mn, Sn, Ni, Co and Mo group and as anion-producing acids at least one acid or one salt of the citric, tartaric, maleic, succinic fumaric, adipic, acetic, lactic,malonic,phthalic, sulfuric, selenic, phosphoric, fluoboric, formic, sulfamic, sulfosalicylic, phenolsulfonic acids group.

3. Method according to claim 2 in which in the said colouring electrolyte at least two different acids or their salts are used as anion-producing acids or as acid radicals.

4. Method according to claim 3 in which the said colouring electrolyte contains at least Cu ions and tin ions, and as anion-producing acids at least one acid or one salt of the acids of the citric, tartaric, maleic, succinic, fumaric, adipic, acetic, lactic, malonic, phthalic acids group and at least one further acid or salt of an acid of the sulfuric, selenic, phosphoric, fluoboric, formic, su'lfamic, sulfo-salicylic, sulfanic, phenosulfonic acids group.

'5. Method for colouring previously anodised protective layers on aluminium and aluminium base alloys in various colour sequences by means of electrolytic .treatment of the protective layers with alternating current under the application of counter electrodes in acidiferous electrolytes containing colouring-metal salts,

7 8 in which a single colouring electrolyte is used containelectrolyte is used containing: w elts f a s tmq 919Pfllli" EEQEQUKDJSDEQQD 5 p to 20 g/l tin-"sulfate one renders a different colour from the other or others 5 up to 10 g/l CuSO 5 H and wherein in this same electrolyte these different colup to 20 g/l citric acid ours are obtained by varying the value of the electrical 5 5 up to 20 g/l sulfuric acid cone, voltage for the electrolytic treatment, wherein a co- 7. Method according to claim 6 in which a colouring louring electrolyte is used containing: electrolyte is used containing 0.5 up to 100 g/l tin-ll-sulfate 13 up to 17 g/l tin-ll-sulfate 0.5 up to 70 g/l CuSO 5 H 0 5 up to g/l CuSO; 5 H O 0.5 up to 100 g/l citric acid I 8 up to l2 g/l citric acid 0.5 up to 100 g/l sulfuric acid cone. 8 up to 12 g/l sulfuric acid. 6. Method according to claim 5 in which a colouring 

2. Method according to claim 1, in which said colouring electrolyte contains at least two kinds of differently colouring ions of the metals in the Cu, Ag, Cd, Mn, Sn, Ni, Co and Mo group and as anion-producing acids at least one acid or one salt of the citric, tartaric, maleic, succinic, fumaric, adipic, acetic, lactic, malonic, phthalic, sulfuric, selenic, phosphoric, fluoboric, formic, sulfamic, sulfosalicylic, phenolsulfonic acids group.
 3. Method according to claim 2 in which in the said colouring electrolyte at least two different acids or their salts are used as anion-producing acids or as acid radicals.
 4. Method according to claim 3 in which the said colouring electrolyte contains at least Cu ions and tin ions, and as anion-producing acids at least one acid or one salt of the acids of the citric, tartaric, maleic, succinic, fumaric, adipic, acetic, lactic, malonic, phthalic acids group and at least one further acid or salt of an acid of the sulfuric, selenic, phosphoric, fLuoboric, formic, sulfamic, sulfo-salicylic, sulfanic, phenosulfonic acids group.
 5. Method for colouring previously anodised protective layers on aluminium and aluminium base alloys in various colour sequences by means of electrolytic treatment of the protective layers with alternating current under the application of counter electrodes in acidiferous electrolytes containing colouring-metal salts, in which a single colouring electrolyte is used containing salts of at least two colouring metals of which each one renders a different colour from the other or others and wherein in this same electrolyte these different colours are obtained by varying the value of the electrical voltage for the electrolytic treatment, wherein a colouring electrolyte is used containing: 0.5 up to 100 g/l tin-II-sulfate 0.5 up to 70 g/l CuSO4 . 5 H2O 0.5 up to 100 g/l citric acid 0.5 up to 100 g/l sulfuric acid conc.
 6. Method according to claim 5 in which a colouring electrolyte is used containing: 5 up to 20 g/l tin-II-sulfate 5 up to 10 g/l CuSO4 . 5 H2O 5 up to 20 g/l citric acid 5 up to 20 g/l sulfuric acid conc.
 7. Method according to claim 6 in which a colouring electrolyte is used containing 13 up to 17 g/l tin-II-sulfate 5 up to 10 g/l CuSO4 . 5 H2O 8 up to 12 g/l citric acid 8 up to 12 g/l sulfuric acid. 